As for a structure of microporous membrane, there are an asymmetric structure (asymmetric membrane) and a symmetric structure (symmetric membrane). An asymmetric membrane is a membrane where the cross-sectional configuration of the membrane at the surface and the underside are not symmetric (left in FIG. 25), and a symmetric membrane is a membrane where the cross-sectional configuration of the membrane at the surface and the underside are symmetric (right in FIG. 25). In an asymmetric membrane, the surface of the membrane consists of a minutely thin layer (functional layer) referred to as a skin layer (or compact layer), and the other major portion consists of a support layer which supports the skin layer. An asymmetric membrane is a single layer, and formed of an extremely thin skin layer on its surface and a support layer made of the same material as the skin layer which supports the skin layer. Conventionally, there have been measures in an asymmetric membrane when used as a filtration membrane, such as reducing the thickness of this skin layer (functional layer) or making a uniform layer to reduce filtration resistance (such as resistance to water flow).
As an example of an asymmetric membrane, Patent Document 1 discloses a porous resin layer of a single layer made of polyvinylidene fluoride. In FIG. 1, FIG. 3, FIG. 5, FIG. 7 and FIG. 9, the structural photos of the surface of porous resin layers made of polyvinylidene fluoride (i.e. the skin layer surface) are disclosed, and in FIG. 2, FIG. 4, FIG. 6, FIG. 8 and FIG. 10, the structural photos of the cross-section of the porous resin layers are disclosed.
However, since polyvinylidene fluoride is a polymer which is mechanically, thermally, and chemically stable, there has been a problem that it is less likely to be processed into microporous membranes. Therefore, pore formation methods of a membrane made of polyvinylidene fluoride involve forming cracks in the surface of the skin layer of the asymmetric membrane, or the use of pores formed by spherulites which are connected to each other in a skin layer. However, by either way, sufficient pores have not been provided, and high permeability (liquid permeability and the like) has not been achieved.